Did you ever wonder how researchers determine levels of confidence? Typically this research is perform using a population of college students volunteer to take part in psychological tests (sometimes a requirement for a psychology course). To measure confidence, students take a general knowledge test and estimate how well they did. Most of the students are not very accurate at guessing their performance on the tests. Typically 40-50% typically are over confident, and 30 to 40 % are under confident, and the small remaining percentage accurately estimate how well they have answered the questions.
Of course the whole purpose of the test is not to find out how many over confident students exist, but to find the correlation between over or under confidence and some other trait. Sometimes population are chosen based on this other trait, such as research involving men and women with similar grades in same major. These studies have revealed that men on the average are overconfident; while women on the average are underconfident. 
Often research includes additional personality assessments tests to determine personality traits that lead to overconfidence. Assessments based on the five factor model (commonly called the “Big Five”) are used with great frequency. These five factors are: openness to experience, conscientiousness, extraversion, agreeableness, and neuroticism. The acronym OCEAN is used to make them easy to remember. For many years the first four traits in this list were considered positive, and neuroticism was considered negative. Although most psychologists now concede that neither extraversion nor introversion are positive or negative; they are simply different.
According to research combining the test of confidence and a five factor analysis, high scores in extraversion significantly predicted overconfidence. Of course, that doesn’t mean that all extroverts are overconfident and introverts are underconfident, but there is a greater chance of these traits appearing together, although not a large one. There is also a mild correlation between agreeableness and overconfidence. The people who rank higher in both extraversion and agreeableness showed the most significant correlation to being over confident compared to others. 
What you must realize first is that the five factor analysis is almost always a self-reported test. It is not surprising to see extraversion correlated with over-confidence. Extraverts tend to measure themselves higher in confidence; while on the test of general knowledge, they were lower in accurately predicting how well they did.  However it is interesting to note that their inaccuracy tended to be biased to make them look better rather than make them look worse.
Have you wondered what does agreeableness have to do with being overconfident? It would seem odd that students that characterize themselves as being friendly, cooperative, good-natured, sociable and nurturing would also inflate reports of their abilities. Agreeableness is seen as a positive characteristics. Perhaps people who want to appear better to others grade themselves higher in the area of agreeableness, even if they aren’t as agreeable as the students sitting next to them. Those extraverts who tended to lack accuracy in predicting how well they did, may also lack accuracy in defining their own agreeableness. In both cases motivation is to appear better than they actually are. Perhaps they even believed their own inaccurate report of themselves. It is easy to do when the rest of us take these self-reported qualities at face value without examining them.